Lithium-ion (Li-ion) batteries are a type of rechargeable battery which produce energy from electrochemical reactions. In a typical lithium ion battery, the cell may include a positive electrode, a negative electrode, an ionic electrolyte solution that supports the movement of ions back and forth between the two electrodes, and a porous separator which allows ion movement between the electrodes and ensures that the two electrodes do not touch.
Li-ion batteries may comprise metal oxides for the positive electrode (herein also referred to as a cathode) and carbon/graphite for the negative electrode (herein also referred to as an anode), and a salt in an organic solvent, typically a lithium salt, as the ionic electrolyte solution. During charge the anode intercalates lithium ions from the cathode and during discharge releases the ions back to the cathode. Recently, lithium metal phosphates, for example lithium iron phosphates, have found use as a cathode electroactive material.
The use of lithium iron phosphate (LFP) provides a next generation replacement for the more hazardous lithium cobalt oxide that is currently used in commercial lithium ion batteries. Li-ion batteries using LFP based cathode materials may currently be found in cordless hand tools and on-board UPS devices. Battery packs have recently been demonstrated for transportation including aviation and electric vehicles as well as plug-in hybrid electric vehicle automobiles and buses.
The characteristics for current LFP materials for use in batteries are often different from or in contradiction with those for other intended purposes. Impurities which may be present due to the synthesis may be detrimental to Li-ion batteries. Furthermore, different batches during synthesis of commercially available LFP materials may often have inconsistent properties. Thus, an LFP material with carefully controlled characteristics is needed which provides consistent and desirable properties for use in Li-ion batteries.
Current LFP materials for use in Li-ion batteries are synthesized from various starting reagents and have a range of characteristics. For example, in U.S. Pat. No. 8,541,136 (Beck et al) provides an LFP material which includes excess lithium and a surface area of 45.5 m2/g to improve discharge rate capabilities. In another example, US 2011/006829 (Beck et al) provides a high purity crystalline phase LFP which is synthesized from a high purity crystalline ferric phosphate material, hereby incorporated by reference for all purposes.
However, the inventors herein have recognized potential issues with the current generation of LFP based cathode materials. The current LFP materials may have limited use in extreme temperature environments, such as exposure to temperature at or below 0° C., as the energy of the Li-ion battery may be too low. Thus, the LFP materials for use in Li-ion batteries need improvements in energy in extreme temperature environments to be used in a broader range of applications. Further, it was recognized that improvements with regards to impedance, power during cold cranking, high rate capacity retention, and charge transfer resistance would improve the current generation of LFP based cathode materials.
One potential approach as found by the inventors to at least address some of the above issues includes synthesizing a plate-shaped spheniscidite precursor which can be used to produce an improved electrode material, also referred to as the LFP material. The plate-shaped precursor may be formed as a single-phase material, and, in some embodiments, may have a surface area in a range of 20 m2/g to 25 m2/g, as disclosed in U.S. Provisional Patent Application No. 61/949,596, entitled “HIGH-POWER ELECTRODE MATERIALS,” filed Mar. 7, 2014, the entire contents of which are hereby incorporated by reference for all purposes.
The LFP material formed from the plate-shaped spheniscidite precursor, herein also referred to as an ammonium iron phosphate precursor, may comprise crystalline primary and secondary particles. The primary particles may have a particle size between about 20 nm to about 80 nm. The secondary particles may have a d50 particle size in the range of 5 microns to 13 microns. In some examples, the secondary particles may have a surface area of in a range of 25 m2/g to 35 m2/g. Further, the secondary particles may have a tap density from about 0.8 g/mL to 1.4 g/mL.
In some examples, the LFP material may contain less than about 5 weight percent of any additional phase that does not substantially store ions. Further, the LFP material, in some embodiments, may have a carbon percentage in the range of 2.1% to 2.5%.
As provided in detail in the description below, the disclosed LFP material may provide improved battery properties in extreme temperature environments. For example, the LFP material may have improved capacity at low temperature, wherein the low temperature may be at or below 0° C. Moreover, the LFP material provides for improved impedance, increased power during cold cranking, increased high rate capacity retention, and improved charge transfer resistance.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.